Recent findings that geldanamycin, an ansamycin isolated from culture filtrates of Streptomyces hygroscopicus, inhibits tyrosine kinase and also exhibits in vitro activity against prostate cancer cell lines are largely responsible for current interest in developing the compound as an antitumor agent. When evaluated in the NCI in vitro antitumor screen, the averaged log of the mid molar concentrations for all cell lines in the panel were: GI50 = -6.82, TGI = -5.57, and LC5O = -4.82. Methods of handling solutions of the compound were developed; complete protection from exposure to light is required since photolytic degradation occurs in most solvent systems. Murine intravenous pharmacokinetic studies afforded plasma profiles exhibiting triexponential decay of the parent drug and rapidly elimination from the body. Low apparent volumes of distribution suggest that the compound is highly bound to plasma proteins relative to its binding by peripheral tissues.